In recent years, silicon modulators have attracted a lot of attention, due to their characteristics of easy integration, low power consumption, CMOS-process compatibility, and relatively smaller size. These benefits are keys to reducing the footprint and power consumption of optical transceiver modules for long-haul and metro telecommunication. In one approach, a MOS-structure based silicon modulator may achieve high speed modulation, benefiting from electro-optic effect. The active region may be 500-μm in length, rather small compared with traditional lithium-niobate (LiNbO3) Mach-Zehnder modulator. Meanwhile, the driving peak-to-peak voltage may be as small as 1.2V, exhibiting 9 dB extinction ratio.
However, in CMOS process, the poly-silicon layer is utilized as the gate layer of the optical waveguide, where high propagation loss is induced due to the absorption and scattering losses of grain boundaries, which results in a high insertion loss. Meanwhile, 100G long-haul coherent transmission has very high requirement of modulator extinction ratio performance, thus the length of MOS-structure silicon modulator has to be extended to achieve the high extinction ratio. This is because higher driving voltage is not a feasible method given the risk of oxide breakdown at higher voltage.